1. Field of the Invention
Curable compositions, such as benzoxazine-based ones, are useful in applications within the aerospace industry, such as for example as a heat curable composition for use as a matrix resin in advanced processes, such as resin transfer molding, vacuum assisted transfer molding and resin film infusion, and their use in such advanced processes form the basis of the present invention.
2. Brief Description of Related Technology
Epoxy resins with various hardeners have been used extensively in the aerospace industry, both as adhesives and as matrix resins for use in prepreg assembly with a variety of substrates.
Blends of epoxy resins and benzoxazines are known. See e.g. U.S. Pat. No. 4,607,091 (Schreiber), U.S. Pat. No. 5,021,484 (Schreiber), U.S. Pat. No. 5,200,452 (Schreiber), and U.S. Pat. No. 5,445,911 (Schreiber). These blends appear to be potentially useful in the electronics industry as the epoxy resins can reduce the melt viscosity of benzoxazines allowing for the use of higher filler loading while maintaining a processable viscosity. However, epoxy resins oftentimes undesirably increase the temperature at which benzoxazines polymerize.
Ternary blends of epoxy resins, benzoxazines and phenolic resins are also known. See U.S. Pat. No. 6,207,786 (Ishida), and S. Rimdusit and H. Ishida, “Development of new class of electronic packaging materials based on ternary system of benzoxazine, epoxy, and phenolic resin,” Polymer, 41, 7941-49 (2000).
Resin transfer molding (“RTM”) is a process by which a resin—conventionally and predominately, epoxy-based resin systems and maleimide-based systems—is pumped at low viscosities and under pressure into a closed mold die set containing a preform of dry fabric. The resin infuses into the preform to make a fiber-reinforced composite article. The RTM process can be used to produce at low cost composite parts that are complex in shape. These parts typically require continuous fiber reinforcement along with inside mold line and outside mold line controlled surfaces.
Fiber-reinforced composite articles may be manufactured from vacuum assisted resin transfer molding (“VaRTM”), like RTM. In contrast to RTM, VaRTM employs an open mold and places the system under a vacuum to assist the resin infusion process.
Resin film infusion (“RFI”), like RTM, infuses a resin into a preform placed in a mold. Here, however, the resin is in the form of a film, which is placed in the mold together with the preform. U.S. Pat. No. 5,902,535 speaks to RFI molds and processes, and is expressly incorporated herein by reference.
The matrix resin used in the RTM and VaRTM advanced prossesses should desirably have a low injection viscosity to allow complete wetting and infusion of the preform.
Bismaleimide-based resins for RTM and RFI processes are known, and examples of which are described in U.S. Pat. Nos. 5,955,566 and 6,313,248.
And, two component epoxy resin compositions have been used, where the epoxy resin and the hardener components are combined immediately prior to use. One component epoxy resin compositions oftentimes must be stored at controlled low temperatures to prevent premature cross-linking reactions and to extend storage life. Otherwise, the viscosities of such one component epoxy resin compositions would build far too quickly, thus rendering their working life unsuitable (or at least not desirable) from a commercial standpoint.
Notwithstanding the state of the technology, there is a need for other resin systems to be used in these advanced processes, particularly a resin system with improved performance properties. And to date there has been no disclosure, teaching or suggestion to prepare a heat curable composition either as a matrix resin or in film form based on benzoxazine-containing compositions for these advanced processes.